Sure thing, let's get straight to it. We start with 840 and we're going to multiply it by 125, which means adding 840 to itself 125 times.
Step 1: 0 + 840 = 840
Step 2: 840 + 840 = 1680
Step 3: 1680 + 840 = 2520
Step 4: 2520 + 840 = 3360
Step 5: 3360 + 840 = 4200
Step 6: 4200 + 840 = 5040
Step 7: 5040 + 840 = 5880
Step 8: 5880 + 840 = 6720
Step 9: 6720 + 840 = 7560
Step 10: 7560 + 840 = 8400
Step 11: 8400 + 840 = 9240
Step 12: 9240 + 840 = 10080
Step 13: 10080 + 840 = 10920
Step 14: 10920 + 840 = 11760
Step 15: 11760 + 840 = 12600
Step 16: 12600 + 840 = 13440
Step 17: 13440 + 840 = 14280
Step 18: 14280 + 840 = 15120
Step 19: 15120 + 840 = 15960
Step 20: 15960 + 840 = 16800
Step 21: 16800 + 840 = 17640
Step 22: 17640 + 840 = 18480
Step 23: 18480 + 840 = 19320
Step 24: 19320 + 840 = 20160
Step 25: 20160 + 840 = 21000
Step 26: 21000 + 840 = 21840
Step 27: 21840 + 840 = 22680
Step 28: 22680 + 840 = 23520
Step 29: 23520 + 840 = 24360
Step 30: 24360 + 840 = 25200
Step 31: 25200 + 840 = 26040
Step 32: 26040 + 840 = 26880
Step 33: 26880 + 840 = 27720
Step 34: 27720 + 840 = 28560
Step 35: 28560 + 840 = 29400
Step 36: 29400 + 840 = 30240
Step 37: 30240 + 840 = 31080
Step 38: 31080 + 840 = 31920
Step 39: 31920 + 840 = 32760
Step 40: 32760 + 840 = 33600
Step 41: 33600 + 840 = 34440
Step 42: 34440 + 840 = 35280
Step 43: 35280 + 840 = 36120
Step 44: 36120 + 840 = 36960
Step 45: 36960 + 840 = 37800
Step 46: 37800 + 840 = 38640
Step 47: 38640 + 840 = 39480
Step 48: 39480 + 840 = 40320
Step 49: 40320 + 840 = 41160
Step 50: 41160 + 840 = 42000
Step 51: 42000 + 840 = 42840
Step 52: 42840 + 840 = 43680
Step 53: 43680 + 840 = 44520
Step 54: 44520 + 840 = 45360
Step 55: 45360 + 840 = 46200
Step 56: 46200 + 840 = 47040
Step 57: 47040 + 840 = 47880
Step 58: 47880 + 840 = 48720
Step 59: 48720 + 840 = 49560
Step 60: 49560 + 840 = 50400
Step 61: 50400 + 840 = 51240
Step 62: 51240 + 840 = 52080
Step 63: 52080 + 840 = 52920
Step 64: 52920 + 840 = 53760
Step 65: 53760 + 840 = 54600
Step 66: 54600 + 840 = 55440
Step 67: 55440 + 840 = 56280
Step 68: 56280 + 840 = 57120
Step 69: 57120 + 840 = 57960
Step 70: 57960 + 840 = 58800
Step 71: 58800 + 840 = 59640
Step 72: 59640 + 840 = 60480
Step 73: 60480 + 840 = 61320
Step 74: 61320 + 840 = 62160
Step 75: 62160 + 840 = 63000
Step 76: 63000 + 840 = 63840
Step 77: 63840 + 840 = 64680
Step 78: 64680 + 840 = 65520
Step 79: 65520 + 840 = 66360
Step 80: 66360 + 840 = 67200
Step 81: 67200 + 840 = 68040
Step 82: 68040 + 840 = 68880
Step 83: 68880 + 840 = 69720
Step 84: 69720 + 840 = 70560
Step 85: 70560 + 840 = 71400
Step 86: 71400 + 840 = 72240
Step 87: 72240 + 840 = 73080
Step 88: 73080 + 840 = 73920
Step 89: 73920 + 840 = 74760
Step 90: 74760 + 840 = 75600
Step 91: 75600 + 840 = 76440
Step 92: 76440 + 840 = 77280
Step 93: 77280 + 840 = 78120
Step 94: 78120 + 840 = 78960
Step 95: 78960 + 840 = 79800
Step 96: 79800 + 840 = 80640
Step 97: 80640 + 840 = 81480
Step 98: 81480 + 840 = 82320
Step 99: 82320 + 840 = 83160
Step 100: 83160 + 840 = 84000
Step 101: 84000 + 840 = 84840
Step 102: 84840 + 840 = 85680
Step 103: 85680 + 840 = 86520
Step 104: 86520 + 840 = 87360
Step 105: 87360 + 840 = 88200
Step 106: 88200 + 840 = 89040
Step 107: 89040 + 840 = 89880
Step 108: 89880 + 840 = 90720
Step 109: 90720 + 840 = 91560
Step 110: 91560 + 840 = 92400
Step 111: 92400 + 840 = 93240
Step 112: 93240 + 840 = 94080
Step 113: 94080 + 840 = 94920
Step 114: 94920 + 840 = 95760
Step 115: 95760 + 840 = 96600
Step 116: 96600 + 840 = 97440
Step 117: 97440 + 840 = 98280
Step 118: 98280 + 840 = 99120
Step 119: 99120 + 840 = 99960
Step 120: 99960 + 840 = 100800
Step 121: 100800 + 840 = 101640
Step 122: 101640 + 840 = 102480
Step 123: 102480 + 840 = 103320
Step 124: 103320 + 840 = 104160
Step 125: 104160 + 840 = 105000

So, 840*125 = 105000